Trophic Responses of the Asian Reservoir to Long-Term Seasonal and Interannual Dynamic Monsoon

Abstract

The main objectives of the study were to determine the trophic response of the temperate reservoir to seasonal and interannual variabilities of monsoon inorganic solids and nutrients along the gradients of the morphologically complex Asian reservoir using long-term datasets between 2000–2018. Nutrient regime (total nitrogen—TN, total phosphorus—TP), total suspended solids (TSS), and chlorophyll-a (CHL-a) were primarily affected by an intensity of summer monsoon and the longitudinal structure of riverine (Rz), transitional (Tz), and lacustrine (Lz) zone. The reservoir is a nitrogen-rich system and the phosphorus content of the water was relatively low, and it had low mean N:P ratios (<40), implying a P-limiting system. The Lz was a highly P-limited zone in comparison to Rz and Tz zone during both drought (2015) and flood year (2011). The TP content was higher in the mainstem (S3) than the embankment (S4 and S6) of the reservoir due to the monsoon river inputs of the nutrients. Nonparametric Mann–Kendall tests indicated that TP decreased over the long-term years in the Rz, while it did not show any trend in Tz, Lz, IT1, and IT2. TN showed an increasing trend in Rz, Tz, Lz, and IT2 except for IT1. The empirical regression model for chlorophyll nutrients showed that CHL-a had a strong positive relationship with TP (R2 = 0.67, p < 0.01) than TN (R2 = 0.06, p < 0.01), supporting the view that algal growth in lentic systems responds to TP enrichment and TP may provide a reliable basis for predicting algal biomass. The seasonality of CHL-a and TP showed a monomodal pattern and indicates that summer TP influences summer algal growth in Tz, Lz, and IT2. The water clarity (SD) of the reservoir was significantly (p < 0.01) influenced by TP (R2 = 0.62), TSS (R2 = 0.67), and CHL-a (R2 = 0.68) rather than TN (R2 = 0.10). The non-algal light attenuation coefficient (Kna) was determined mainly by suspended solids and the monsoon hydrology. The trophic state was much higher when assessments were based on TSI (CHL-a) than on TSI (TP) and TSI (SD). TSI (CHL-a) indicated the eutrophic state of the reservoirs except for the zone of Lz during the premonsoon season. Analysis of trophic state index deviation (TSID) suggested that the blue-green algae dominated the algal community, and the effects of non-algal turbidity and zooplankton grazing were minor in the reservoir.

Long-Term Interannual and Seasonal Links between the Nutrient Regime, Sestonic Chlorophyll and Dominant Bluegreen Algae under the Varying Intensity of Monsoon Precipitation in a Drinking Water Reservoir

“…Although CHL-a disclosed a similar development as TP, it is essential to mention an overall decline in the annual CHL-a levels during the study period, mostly post 2015 drought year it is showed a sharp decline. A valid reason for this decline could be the successive increase in annual rainfall that could have washed out most of the algal CHL-a [ 12 ]. TSS disclosed varied loads to the yearly fluctuating precipitation.…”

“…These results approximate the spatial dynamics of nutrients, algal CHL-a and organic pollution indicators in reservoir zones and water intake tower sites. This is important to mention the due to the almost no relation with TN, the chances of collinearity are minor, and the system is strongly P-limited [ 12 , 35 , 58 ].…”

“…The Lz, however, indicates the reservoir region representing the down lake area adjacent to the dam section (characterized by lower flow), and the Tz represents the intermediary features between those of Rz and Tz (relatively wider and intermediate water flow). The DR’s major feeding tributaries include Seowha, Bouchoung, Soung, Youngdong, Whebuk, and Janggae streams [ 12 , 35 ]. The site-based land use pattern could be described as S 1 (40% rural area, 50% crop farming, 10% livestock), S 2 (90% urban, 10% crop farming and openly subjected to the wastewater disposal plant (WWTP), S 3 (45% crop farming, forest), S 4 , S 5 , and S 6 (78% forest area and 22% rural area).…”

“…Several are located near large metropolitans and serve as sources of drinking water supply and help mitigate floods during intensive monsoon rainfall events. However, the large reservoirs are under severe water quality degradation and nutrient enrichment, which jeopardize human and ecosystem services [ 12 ].…”

Seasonal and Long-Term Connections between Trophic Status, Sestonic Chlorophyll, Nutrients, Organic Matter, and Monsoon Rainfall in a Multipurpose Reservoir

“…The findings showed the nutrient loadings from watersheds caused by extreme rainfall events that increased the algal CHL-a growth [21,[54][55][56]. The increasing TSS levels, BOD, TP, and CHL-a during the flood years declined, indicating intensive rainfall mediated dilution [20,46,57,58].…”